Field of the Invention
Embodiments of the present disclosure relate generally to anti-islanding of distributed power generation systems, and, in particular, to detecting a grid impedance change for anti-islanding.
Description of the Related Art
Solar panels, or photovoltaic (PV) modules, convert energy from sunlight received into direct current (DC). The PV modules cannot store the electrical energy they produce, so the energy must either be dispersed to an energy storage system, such as a battery or pumped hydroelectricity storage, or dispersed by a load. One option to use the energy produced is to employ inverters to convert the DC current into an alternating current (AC) and couple the AC current to the commercial AC power grid. The power produced by such a distributed generation (DG) system can then be sold to the commercial power company.
Under some conditions, a grid-connected DG system may become disconnected from the utility grid, resulting in a potentially dangerous condition known as “islanding”. During islanding, the utility cannot control voltage and frequency in the DG system island, creating the possibility of damage to customer equipment coupled to the island. Additionally, an island may create a hazard for utility line workers or the general public by causing a line to remain energized while it is assumed that the line has been disconnected from all energy sources. In order to mitigate the potential hazards of islanding, relevant standards require that inverters in a DG system detect the loss of the utility grid and shut down the inverter. As part of detecting an islanding condition, some standards may require that the inverter be able to detect an instantaneous impedance change on the grid within a certain time period, for example an instantaneous 1 ohm impedance change on the grid within 5 seconds. As such, all commercially available inverters must be equipped with such inverter-based anti-islanding capability.
Therefore, there is a need in the art for a method and apparatus for efficient detection of grid impedance changes by a grid-coupled inverter.